Many surgical interventions require the placement of a medical device into the body. While necessary and beneficial for treating a variety of medical conditions, the placement of metal or polymeric devices in the body gives rise to numerous complications. Some of these complications include: increased risk of infection; initiation of a foreign body response resulting in inflammation and fibrous encapsulation; and initiation of a wound healing response resulting in hyperplasia and restenosis. These, and other complications must be dealt with when introducing a metal or polymeric device into the body.
One approach to reducing the potential harmful effects of such an introduction is to attempt to provide a more biocompatible implantable device. While there are several methods available to improve the biocompatibility of implantable devices, one method which has met with limited success is to provide the device with the ability to deliver bioactive compounds to the vicinity of the implant. By so doing, some of the harmful effects associated with the implantation of medical devices can be diminished. Thus, for example, antibiotics can be released from the surface of the device to minimize the possibility of infection, and anti-proliferative drugs can be released to inhibit hyperplasia. Another benefit to the local release of bioactive agents is the avoidance of toxic concentrations of drugs which are sometimes necessary, when given systemically, to achieve therapeutic concentrations at the site where they are needed.
Although the potential benefits expected from the use of medical devices capable of releasing pharmaceutical agents from their surfaces is great, the development of such medical devices has been slow. This development has been hampered by the many challenges that need to be successfully overcome when undertaking said development. Some of these challenges are: 1) the requirement, in some instances, for long term release of bioactive agents; 2) the need for a biocompatible, non-inflammatory device surface; 3) the need for significant durability, particularly with devices that undergo flexion and/or expansion when being implanted or used in the body; 4) concerns regarding processability, to enable the device to be manufactured in an economically viable and reproducible manner; and 5) the requirement that the finished device be sterilizable using conventional methods.
Several implantable medical devices capable of delivering medicinal agents have been described. Several patents are directed to devices utilizing biodegradable or bioresorbable polymers as drug containing and releasing coatings, including Tang et al , U.S. Pat. No. 4,916,193 and MacGregor, U.S. Pat. No. 4,994,071. Other patents are directed to the formation of a drug containing hydrogel on the surface of an implantable medical device, these include Amiden et al, U.S. Pat. No. 5,221,698 and Sahatjian, U.S. Pat. No. 5,304,121. Still other patents describe methods for preparing coated intravascular stents via application of polymer solutions containing dispersed therapeutic material to the stent surface followed by evaporation of the solvent. This method is described in Berg et al, U.S. Pat. No. 5,464,650.
However, there remain significant problems to be overcome in order to provide a therapeutically significant amount of a bioactive compound on the surface of the implantable medical device. This is particularly true when the coating composition must be kept on the device in the course of flexion and/or expansion of the device during implantation or use. It is also desirable to have a facile and easily processable method of controlling the rate of bioactive release from the surface of the device.
Although a variety of hydrophobic polymers have previously been described for use as drug release coatings, Applicant has found that only a small number possess the physical characteristics that would render them useful for implantable medical devices which undergo flexion and/or expansion upon implantation. Many polymers which demonstrate good drug release characteristics, when used alone as drug delivery vehicles, provide coatings that are too brittle to be used on devices which undergo flexion and/or expansion. Other polymers can provoke an inflammatory response when implanted. These or other polymers demonstrate good drug release characteristics for one drug but very poor characteristics for another.
Some polymers show good durability and flexibility characteristics when applied to devices without drug, but lose these favorable characteristics when drug is added. Furthermore, often times the higher the concentration of drugs or the thicker the application of polymer to the device surface, the poorer the physical characteristics of the polymer become. It has been very difficult to identify a polymer which provides the proper physical characteristics in the presence of drugs and one in which the drug delivery rate can be controlled by altering the concentration of the drug in the polymer or the thickness of the polymer layer.
There remains a need, therefore, for an implantable medical device that can undergo flexion and/or expansion upon implantation, and that is also capable of delivering a therapeutically significant amount of a pharmaceutical agent or agents from the surface of the device.